A standard hydropneumatic blind riveter has a housing forming an air chamber, a liquid-filled hydraulic chamber adjacent the air chamber, and a liquid-filled working chamber communicating permanently with the hydraulic chamber. A working piston is provided in the working chamber and carries a chuck for grasping a mandrel of a blind rivet. This working piston is displaceable in the working chamber between a ready position wherein a mandrel can be fitted into the chuck and an actuated position wherein the chuck is retracted, displacement from the former to the latter position upsetting the rivet and pulling the mandrel from it. An air piston is displaceable between a front and a rear position in the air chamber and subdivides it into a front compartment and a rear compartment. The housing is formed at the front end of the front compartment with a vent and a hydraulic piston in the hydraulic chamber is fixed to and therefore jointly displaceable with the air piston, but between an advanced position pressurizing the hydraulic chamber and corresponding to the front position of the air piston and a retracted position corresponding to the rear position thereof. A valve is provided for admitting pressurized air into the rear compartment and thereby displacing the air and hydraulic pistons into the respective front and advanced positions, while simultaneously displacing the working piston into the actuated position. A spring braced between the housing and the working piston urges same into its ready position. Thus displacement of the air piston by air pressure hydraulically displaces the working piston to upset a rivet grasped by its chuck. When pressurization through the valve of the rear compartment is stopped, this pressure can drain off, normally through the valve which is made with loose tolerances to permit such leakage.
Earlier U.S. Pat. No. 4,050,285 of K. Bosch and M. Fritzenschaft proposes a modification of this system, aimed at speeding up the return of the chuck to the ready position after a riveting operation. A second valve is provided in a second hole in a wall of the rear compartment and blocks air flow from this compartment when the first valve is actuated to pressurize it, but otherwise is open to allow it to depressurize rapidly. This second valve can merely be a high-pressure check valve which only closes at the high operating pressure of the system, opening as soon as pressure in the rear compartment drops slightly as the first valve closes.
It is known to equip such an arrangement at the top or rear end of the riveting head with a mandrel-catching compartment. Thus once the riveting operation is over the device is shaked, or a new rivet is inserted so its mandrel pushes the pulled-off mandrel in the chuck back through a passage in the working piston to this compartment. Either way such an arrangement has been found to slow down a worker, who is normally required to set rivets at great speed with the machine, since blind rivets are often employed in situations, such as aircraft assembly, where large numbers of rivets form the connection.
Accordingly U.S. Pat. No. 4,281,531 of H. Ehmann, W. Bieber, and H. Baier suggests a pneumatic rivet-core or mandrel disposal system that is carried right on the riveter which otherwise is substantially the same as the one described above. In this arrangement means is provided for aspirating air from the mandrel-catching compartment and thereby for forming an air current through the passage to the chuck so as to suck a mandrel pulled from the rivet body into the compartment. Thus suction is exerted at the storage compartment to eliminate a complicated connection of the pneumatic line to the working tip of the riveter, as is known to blow the mandrel back into the compartment.
More specifically this invention has a cover provided with a jet pump having a high-pressure input connected to the source of compressed air that powers the riveter, a low-pressure intake connected to and opening into the compartment, and an output open to the outside. Thus the high-pressure compressed air used to operate the riveter passes through this jet pump to depressurize the storage compartment and create the pressure differential that sucks the mandrel back through the passage into the compartment once it has been separated from its rivet. This suction is continuously present, so it also serves to secure a fresh rivet to the working tip before it is upset.
With this system there is a constant current of air blowing out of the back of the tool with the concomitant noise. A considerable amount of air passes through the riveter, putting quite some strain on the compressor operating it and wasting energy. The suction created at the tip must be sufficient to aspirate the mandrel after it is pulled off the rivet body, a suction much stronger than that necessary to hold a rivet in place on the tool. In fact such powerful suction is frequently disadvantageous, in that it can pick up foreign bodies or suck a not yet upset rivet out from workpieces into which it has been carefully inserted.